cpuset(9): Add CPU_FOREACH_IS(SET|CLR) and modify consumers to use it

This implementation is faster and doesn't modify the cpuset, so it lets
us avoid some unnecessary copying as well. No functional change
intended.

Reviewed by: cem, kib, jhb
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32029